The use of X-ray imaging systems is well known for use in diverse fields, including utilization in connection with medical diagnosis and/or procedures. Such systems have included fixed-type imaging systems wherein the X-ray source and sensor are maintained in fixed positions to image a body portion within a field of view (FOV) at a scan area (see, for example, U.S. Pat. No. 5,142,557 to Toker et al.) and scan-type imaging systems wherein the X-ray source and/or sensor are moved to image a body portion within a field of view (FOV) at a scan area (see, for example, U.S. Pat. Nos. 4,709,382 to Sones and 4,998,270 to Scheid et al.).
In addition, X-ray imaging systems have also included full field film-type readout units therein an image is recorded on a film cassette or the like (see, for example, U.S. Pat. No. 4,998,270 to Scheid et al.), as well as electronic readout units wherein electrical signals indicative of an image are normally converted to digital signals and the digital signals are then used to display and/or electronically store the image (see, for example, U.S. Pat. Nos. 5,142,557 to Toker et al. and 5,289,520 to Pellegrino et al.).
Such systems normally require a converter, such as a phosphor converter screen, to form and provide light signals responsive, and proportional, to received X-rays passed through the body portion then subjected to X-rays, and electronic readout systems require the converted signals (i.e., the light signals converted from the X-rays) to be coupled, normally through a coupler, such as a fiber optic (OF) coupler, to a sensor, such as a charge coupled device (CCD) or preferably a time delay integrated (TDI) CCD, providing electrical signal outputs responsive to received light signals.
In electronic diagnostic X-ray imaging applications, it has been found to be impractical to attempt to instantaneously image large fields of view since large FOV systems require one or both of very large CCDs or very large fiber optic (OF) reducers, making such sensors impossible, or at least quite expensive, to produce.
While the problem of obtaining a large FOV might be overcome by using lens based systems with large magnification, such systems would be subject to being excessively lossy, requiring an increase in patient dosage of X-rays in order to obtain a satisfactory signal-to-noise ratio (SNR) for the system.
Optically coupled system shortcomings might also be solved, at least in part, by the use of a slit scanner using either one or a multiple number of CCDs working in the time delay integrated (TDI) mode. In general, these TDI-CCDs are bonded to a OF-Reducer on whose front surface an X-ray phosphor is mounted, and this single, or multistage, TDI-CCD-FO-Phosphor assembly is then mechanically scanned while the charge accumulated in the TDI-sensor is manipulated by vertical transport phases synchronous to the mechanical scan. The use of a layer of phosphor over the entirety of a photodiode array without relative movement therebetween is shown, for example, in U.S. Pat. Nos. 4,709,382 to Sones and 4,845,731 to Vidmar et al.
A difficulty arises with respect to the above approach, however, if the phosphor moving under the object, or body portion, to be imaged has an appreciable decay time with respect to the time of motion (a short decay time is required of the X-ray phosphor in order to avoid smear to obtain high modulation within the image). If the decay time is appreciable, then smear, and therefore a significant loss of modulation of the signal (i.e., loss of resolution) is experienced. Since diagnostic X-ray imaging, for example, is of low contrast, any loss of modulation is also a loss of contrast and therefore is unacceptable.
Also, the scanning speed that can be obtained is limited by the X-ray to visible light conversion efficiency of the phosphor and the phosphor converter output decay time. In general, short decay time phosphors have a poor conversion efficiency and poor resolution. Some of these shortcomings, however, might be at least partially overcome by using exotic phosphor systems.
Thus, the reason that high efficiency short decay time X-ray phosphors are needed for TDI-CCD applications is the necessity to move the phosphor with the sensor. If only the sensor is moved and the X-ray phosphor remains stationary, the decay time of the phosphor is immaterial.
Obviously, an X-ray imaging system that does not require movement of the phosphor along with the sensor, thus removing the necessity for short decay time X-ray phosphors (since the decay time of the phosphor would then be immaterial), would be advantageous.